1. The Field of the Invention
The present invention relates to analog integrated circuit design, and more particularly, to a direct conversion receiver that processes constant envelope phase and frequency modulated signals.
2. Background and Related Art
Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (RF) frequency spectrum.
In order to properly interpret the signal, conventional RF receivers extract the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry. In order to perform this extraction, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (“IF”) signal. This intermediate frequency signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called “heterodyne” receiver architectures. Naturally, circuit elements (called “IF components”) are required in order to deal with the intermediate conversion to and from the intermediate frequency.
It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. This is particularly true of wireless RF receivers since those receivers are often portable and run on battery power.
One technology developed in order to reduce RF receiver cost, size, and power consumption is called “direct conversion.” Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often also called “zero-IF,” “synchrodyne,” or “homodyne” receiver architectures.
FIG. 7 illustrates a conventional direct conversion circuit 700 in accordance with the prior art. The circuit 700 includes an antenna 701 which receives the RF modulated signal. The antenna 701 then provides the received signal to an amplifier 702 which amplifies the signal for further processing. The amplifier 702 may be, for example, an RF low noise amplifier.
The amplified signal is then split into two branches, an “in-phase” branch 710, and a “quadrature-phase” branch 720. Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch 710 includes an in-phase mixer 711, and the quadrature-phase branch 720 includes a quadrature-phase mixer 721. A local oscillator 730 provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a 90-degree phase shifter 731 which causes the control signal for the quadrature-phase mixer 721 to be 90 degrees out of phase with the control signal for the in-phase mixer 711.
The signal from the in-phase mixer 711 is then passed through a low pass filter 712 to a baseband amplifier 713 to complete the extraction of the baseband (difference frequency) signal from the received signal as far as the in-phase branch 710 is concerned. Likewise, the signal from the quadrature-phase mixer 721 is passed through a low pass filter 722 to a baseband amplifier 723 to complete the extraction of the baseband (difference frequency) signal as far as the quadrature-phase branch is concerned. The in-phase and quadrature-phase baseband signals are then processed by signal processing circuitry 750.
The direct conversion circuit of FIG. 7 does not convert through an intermediate frequency and thus there are no IF components needed to deal with an intermediate conversion. Consequently, the direct conversion circuit of FIG. 7 is smaller, and requires less power than conventional heterodyne receiver architectures. Furthermore, the direct conversion circuit does not have to deal with image suppression as much as do heterodyne receivers. Accordingly, direct conversion receivers have many advantages over heterodyne receiver architectures. Unfortunately, direct conversion architectures characteristically introduce more DC offset and 1/f noise than do heterodyne receiver architectures thereby limiting dynamic range.
In a direct conversion receiver architecture, as in heterodyne receiver architectures, it is often desirable to implement antenna diversity. Antenna diversity involves the use of more than one antenna to receive a signal to improve the ability to properly receive the signal. When using one antenna to receive a signal, the signal may have actually taken several paths from the transmitter to the receiver, each having a different length. This causes an echo effect that might actually lead to destructive interference between the signals receive from different paths. The use of two or more antennas that are appropriately spaced reduces the degradation due to the echo effect since the echo at one antenna will typically be different than the echo at another, thereby reducing the likelihood that the echo would degrade the signal.
The use of multiple antennas is also helpful to improve signal-to-noise ratio even if there is no echo effect. By coherently adding the two versions of signal together, the signal-to-noise ratio may improve by a factor of the square root of the number of antennas in the diversity receiver system. Conventional antenna diversity systems that perform such coherent adding of the signal consistently add the two signals together without reverting back to a single antenna system when one of the antennas is not picking up a good signal.
Accordingly, a diversity receiver would be advantageous in which the antenna selection is adaptively determined based on the ability of an antenna at any given point in time to properly receive the signal. It would further be advantageous if such a diversity receiver could implement direct conversion receivers, especially if such direct conversion receivers had reduced DC offset and 1/f noise as compared to conventional direct conversion receivers.